1. Field of the Invention
This invention relates to a process of making a sintered molding provided with at least one molybdenum-containing wear-resisting facing, wherein a low-alloy iron powder which is intended to form the base of the molding and a carbon-free metal powder mixture which is intended to form the wear-resisting facing and consists of unalloyed iron particles and unalloyed molybdenum particles are jointly molded in a common mold to form a molding, which is subsequently sintered.
2. Description of the Prior Art
Rocker arms for transmitting motion from a camshaft to a valve, e.g., in an internal combustion engine, have an end face which cooperates with a cam and is subjected to high stresses, which result in a considerable wear. For this reason that surface of the rocker arm which is intended to cooperate with a cam should be provided with a facing which has a high resistance to wear under the loads to be expected and consists of a meterial which is a good match with the material of the camshaft.
German Pat. No. 2,822,902 discloses a simple and inexpensive process of making valve tappets which have a wear-resisting facing, which particularly distinguishes by having a high resistance to wear under sliding friction. In that process said wear-resisting facing is made from a carbon-free powder mixture of unalloyed iron particles and unalloyed molybdenum particles, which mixture is sintered at a relatively high temperature up to 1350.degree. C. For that purpose the metal powder for making the wear-resisting facing and subsequently a low-alloy iron powder for forming the base of the molding are changed into a common mold in which they are jointly molded to form a molding, which is subsequently sintered. But that joint molding may result in a mixing of the two metal powder layers at their interface so that the low-alloy iron powder may enter the layer which is intended to form the wear-resisting facing and the resistance of that facing to wear will thus be reduced. Besides, the joint molding of the two metal powder layers cannot always be controlled to ensure that the wear-resisting facing will have a predetermined thickness. Moreover, the conditions of internal friction in the two metal powder layers are different so that the joint molding will not necessarily result in a compaction of the two powder layers to a unuform density.
In the making of composite moldings by a joint molding of layers of different powders, it is known from published, German Pat. Application No. 3,305,875 to provide a synthetic resin-bonded compact for forming a wear-resisting facing and to place such compact into a mold, in which the compact is then molded and sintered together with a metal powder which has been charged into the mold and is intended to form the base of the molding to be made. Because the compact is bonded by a synthetic resin, that known process cannot be used to make molybdenum-containing, wear-resisting facing as the synthetic resin binder necessarily introduces carbon into the wear-resisting facing, which owing to the presence of such carbon will not have the desired properties.